1. Field of the Invention
The present invention relates to a apparatus using a two-terminal device as a switching device and a method for producing the same.
2. Description of the Related Art
A representative display apparatus replacing CRTs which have been used for a long time is a liquid crystal display apparatus (hereinafter, referred to as the "LCD apparatus"). LCD apparatuses use a liquid crystal layer including liquid crystal molecules as a display medium. Letters and images are displayed by applying a voltage to the liquid crystal layer in order to cause changes in the electrooptic properties of the liquid crystal molecules. In order to display high quality images by providing pixels at a high density, each pixel is supplied with a nonlinear active element (switching device) for driving the LCD apparatus. This system of driving is referred to as the "active matrix driving system". The switching devices are mainly available in two types: two-terminal devices such as MIM (metal insulator metal) elements, diodes, and varistors; and three-terminal devices such as TFTs (thin film transistors) and MOS-FETs (metal oxide semiconductor field effect transistors).
The three-terminal devices have functions as switching devices and are suitable for displaying an image having various tones, for which different pixels are used for each of the different tones. However, the three-terminal devices have some inconveniences in that the complicated production process including repetition of exposure to light can easily cause defects in the obtained devices, thus resulting in low production yields. The two-terminal devices, which have a simpler structure than that of the three-terminal devices, are produced by a simpler method due, for example, to fewer steps of masking required. Accordingly, the production yield of the two-terminal devices is higher than that of the three-terminal devices. For this reason, methods for driving pixels using the two-terminal devices, especially by utilizing a nonlinear part of the operating characteristics of the two-terminal devices, have been actively researched and developed.
There are mainly two methods to use the two-terminal devices in a display apparatus. One is to utilize the nonlinearity of the capacitance of the two-terminal devices, and the other is to utilize the nonlinearity of the electric resistance of the two-terminal devices.
Methods to utilize the nonlinearity of the capacitance of the two-terminal devices for an LCD apparatus are described by Grabmair et al. in Mol. Cryst. Liq. Cryst. 15 (1971) and by Tannas in SID' 73 Symp. Digest (1973). In these publications, ferroelectric liquid crystals are used. However, such methods have not been put into practical use. The reasons include: (1) a large driving voltage is required; and (2) since the dielectric constant of the ferroelectric liquid crystals largely depends on temperature, the characteristics of the resultant LCD apparatus also largely depend on temperature.
Utilization of the nonlinearity of the electric resistance is described by Lechner in Proc. IEEE 59. (1971), by Castleberry in IEEE. Trans. Electron Devices ED-26 (1979), and by Baraff in IEEE. Trans. Electron Devices ED-28 (1981). Lechner uses a diode, Castleberry uses a varistor formed of ZnO, and Baraff uses an MIM element.
A display apparatus using an MIM element includes the following advantages.
(1) The area occupied by an MIM element in one pixel is smaller than the area occupied by a TFT. Accordingly, the ratio of an area of pixels with respect to an image plane (the opening ratio) is larger than in the case of a TFT.
(2) Since the scanning lines and the signal lines do not intersect each other on a substrate as they do in the case of a TFT, there are fewer line defects caused by insufficient insulation at the intersections.
(3) The production process is simpler, e.g., the number of masking steps is smaller than in the case of a TFT. Accordingly, the production yield is higher.
(4) There is no electric current excited by light, which is a problem in TFTs using amorphous silicon, diodes and the like. This eliminates the necessity of shielding the MIM element from external light.
Briefly referring to FIGS. 26 and 27, a conventional display apparatus 50 will be described. FIGS. 26 and 27 show an example of a conventional display apparatus 50 including a conventional MIM element 58 as a switching device. FIG. 26 is a top view of a pixel and the vicinity, thereof in the display apparatus 50; and FIG. 27 is a cross sectional view of the display apparatus 50 shown in FIG. 26 looking along section line 27'--27' in FIG. 26.
The display apparatus 50 includes an insulating substrate 51 formed of glass or the like. A scanning line 52 formed of tantalum (Ta) is on a top surface of the substrate 51. An electrode 56 is branched from the scanning line 52 perpendicularly to the scanning line 52. A surface of the scanning line 52 and a surface of the electrode 56 are anodized so as to be an insulation layer 53. Specifically, the insulation layer 53 is formed of Ta.sub.2 O.sub.5. A rectangular metal layer 54 formed of Ta or the like is on the substrate 51, covering the insulation layer 53. The metal layer 54 is arranged in a direction so as to cross the electrode 56. A generally rectangular pixel electrode 55 as is shown in FIG. 26 is on the substrate 51, covering two ends of the metal layer 54.
The MIM element 58 includes a three-layer structure including the electrode 56, the insulation layer 53, and the metal layer 54. The electrode 56 acts as a first metal layer, the insulation layer 53 acts as an active layer, and the metal layer 54 acts as a second metal layer. Display apparatuses including such a two-terminal device having nonlinear characteristics are now produced as commercial products.
Japanese Patent Publication Nos. 61-32673 and 61-32674 each disclose an LCD apparatus including an MIM element as the two-terminal device.
In order to use the above-described two-terminal device as a switching device, the voltage V applied to an active layer (the insulation layer in the case of an MIM element) between the first metal layer and the second metal layer and the current I flowing between the two metal layers in accordance with the voltage V should have the following I-V characteristic:
(1) The current I rises steeply in accordance with the rise of the voltage V. In other words, the I-V characteristic has a satisfactory steepness.
(2) The absolute value of the current I depends on the absolute value of the applied voltage V and is independent of the polarity (+ or -) of the voltage V. In other words, the I-V characteristic has a satisfactory symmetry in the polarity of the voltage.
For obtaining an image which is entirely uniform in quality, all the pixels in the image plane should have an I-V characteristic excellent both in steepness and symmetry.
FIG. 28 is a graph illustrating a curve representing the I-V characteristic of a conventional MIM element used, for example, in the display apparatus 50. As is apparent from FIG. 28, the I-V characteristic does not show the above-mentioned symmetry. This fact is attributable to the following reasons (1) and (2) below.
(1) The first metal layer is in contact with the insulation layer formed by anodizing the first metal layer. The second metal layer is formed on the insulation layer by sputtering or CVD (chemical vapor deposition). The interface between the first metal layer and the insulation layer and the interface between the second metal layer and the insulation layer are formed by different processes from each other. Accordingly, these two interfaces are in different states.
(2) While the first metal layer is anodized, impurities go into the insulation layer, thus causing the interface between the first metal layer and the insulation layer and the vicinity thereof to have a different dopant concentration from that at the interface between the second metal layer and the insulation layer and the vicinity thereof.
As is described above, the state of the interface between the first metal layer and the insulation layer and the state of the interface between the second metal layer and the insulation layer influence the I-V characteristic of the MIM element. Accordingly, types of metals usable for the MIM element are restricted. Conventionally, where Ta is used for the first metal layer and Ta.sub.2 O.sub.5 is used for the insulation layer, the second metal layer is formed of Cr, Ta or Ti. If the second metal layer is formed of Al, or ITO (indium tin oxide), the symmetry of the I-V characteristic is significantly deteriorated.
The I-V characteristic of the MIM element is expressed by Equations (1), (2) and (3) by Poole-Frenkel current. ##EQU1## where q is the electric charge, n is the carrier density, .mu. is the mobility, .phi. is the depth of the trap, d is the thickness of the insulation layer, T is the temperature, K is the Boltzmann constant, and .di-elect cons. is the dielectric constant.
As is apparent from Equation (1), .beta. which is expressed by Equation (3) indicates the steepness of the I-V characteristic. It is preferable to obtain the highest possible value for .beta.. For example, the value of .beta. is approximately 3 to 4 inclusive in the MIM element including the insulation layer formed of Ta.sub.2 O.sub.5, while the value of .beta. in a varistor, which is a type of the two-terminal device, is approximately 7 to 8 inclusive. This signifies that the MIM element using Ta.sub.2 O.sub.5 for the insulation layer is inferior to the varistor in the I-V characteristic in the steepness.
It is apparent from Equation (3) that the value of .beta. depends on the thickness d of the insulation layer. Therefore, the I-V characteristic changes in accordance with any slight difference in the thickness d. This causes dispersion among the curves representing the I-V characteristic of a plurality of MIM elements for driving respective pixels. As a result, the pixels are put into different display states, thus deteriorating the display quality of the whole image.
Conventional two-terminal devices, for example, the MIM element, also have the following problems in the production process.
In the case where the active layer (for example, the insulation layer 53 in the case of the MIM element 58 in FIG. 27) insufficiently covers the first metal layer (the first electrode) 56, especially on a tapered side surface 57 of the first metal layer 56 and the vicinity thereof, insulation breakdown occurs in the MIM element 58. In this specification, the vicinity of the tapered side surface 57 includes an edge formed at the junction of a top surface 59 of the first metal layer 56 and the tapered side surface 57 and also includes an edge formed at the junction of the tapered side surface 57 and the interface between the first metal layer 56 and the base substrate 51.
For example, in the case where the first metal layer 56 is patterned by etching, insufficient insulation of the tapered side surface 57 and the vicinity thereof adversely influences the performance of the MIM element 58, thereby causing defects and deterioration in the MIM element 58.
In order to solve these problems, some proposals have been made.
One of the proposals is made in Japanese Laid-Open Patent Publication No. 1-270027, which discloses an MIM element as is shown in FIG. 29.
As is shown in FIG. 29, a display apparatus 60 includes a substrate 61. A first metal layer 62 is on a top surface of the substrate 61. A surface of the first metal layer 62 is anodized so as to be an insulation layer 63 (active layer). An insulating intermediate layer 66 is on the substrate 61, covering the insulation layer 63. The intermediate layer 66 has a hole 67 reaching a top flat surface 68 of the insulation layer 63. A second metal layer 64 is on the intermediate layer 66. A pixel electrode 65 is on the intermediate layer 66, covering an end of the second metal layer 64. The second metal layer 64 is in contact with the insulation layer 63 through the hole 67 and also is in contact with the pixel electrode 65. By such a structure, an MIM element 70 includes a flat portion of the first metal layer 62, but excludes a tapered side portion 69 and the vicinity thereof in the first metal layer 62. Accordingly, the performance of the MIM element 70 is protected against adverse influences caused by insufficient insulation of the tapered side portion 69 and the vicinity thereof.
Japanese Laid-Open Patent Publication No. 5-72570 discloses a method for producing an MIM element 80 as is shown in FIG. 30. In FIG. 30, identical elements with those in FIG. 29 bear identical reference numerals therewith.
According to this method, after the first metal layer 62 is formed in a pattern on a top surface of the substrate 61, the insulation layer 63 (active layer) and the intermediate layer 66 are formed by radiating light using the first metal layer 62 as a mask (a photoresist method) from the side of a bottom surface of the substrate 61. As a result, the insulation layer 63 is formed only on a flat top surface 71 of the first metal layer 62. Due to such a structure, the problem of insulation breakdown due to insufficient insulation of a tapered side portion and the vicinity thereof is solved.
However, in order to realize the structure shown in FIG. 29, in which the second metal layer 64 is in contact with the insulation layer 63 and also is in contact with the pixel electrode 65, a part of the intermediate layer 66 and a part of the insulation layer 63 should selectively be etched away. If the insulation layer 63 is formed of a material having a low resistance against chemicals used as an etchant, it is difficult to realize the structure shown in FIG. 29.
In the case where the photoresist method is used as in FIG. 30, the manufacturing precision will be quite limited.
Due to these reasons, it is difficult to put the display apparatus 60 shown in FIG. 29 and the MIM element 80 into practical use.
In the structure shown in FIG. 29, the insulation layer 63 is formed in areas other than the MIM element 70. Accordingly, the leak current has undesirabe increases, so the impedance undesirably changes in accordance with the change in the applied voltage. For these reasons, the display quality declines.